Device for generating thz radiation

ABSTRACT

A device for generating terahertz radiation. The device comprising a dipole generating layer, a coupling block and an extraction block. The coupling block is transparent to laser light and is in contact with the surface of the dipole generating layer to couple light from a laser to the surface of the dipole generating layer, when the device is in use. The extraction block is located in contact with the surface of the dipole generating layer to provide an emission extraction surface. The refractive indices of the dipole forming layer, the coupling block and the extraction block are substantially equal. In this way, the dipole which is generated upon illumination of the dipole generating layer by a laser, has an axis which is not perpendicular to the emission

[0001] This invention relates to the generation and extraction ofterahertz (THz) radiation in semiconductors.

[0002] Emerging applications of coherent THz radiation spectroscopy andimaging are currently limited by the lack of compact, high power THzemitters.

[0003] Surface field THz generation occurs when a semiconductor isilluminated by a sub-picosecond laser pulse with photon energy greaterthan the semiconductor bandgap. The electric field within the surfacedepletion region accelerates electrons and holes in opposite directions.The resulting charge separation, which occurs on a picosecond timescale, forms a dipole that emits a coherent THz transient.

[0004] THz generation occurs in p-i-n diodes when the device isilluminated with sub-picosecond laser pulses, when the photon energy isgreater than the bandgap energy of the semiconductor making up the i(intrinsic) region. The electric field within the i region of the p-i-naccelerates the photogenerated charges. This electric field can beincreased or decreased by applying a voltage between the n-region andthe p-region. By adjusting the voltage the maximum output frequency ofthe THz radiation can be tuned.

[0005] The spot size, to which the excitation laser is focussed,determines the lateral extent of the THz dipole. If this spot size issmall (<100 microns), diffraction effects are important, and the THzradiation is emitted over a broad range of angles. To produce a directedTHz beam it is necessary to use a collimated laser beam with beam-waistlarger than the THz wavelength (typically approximately 250 microns).The dipole then emits strongly only in the direction of the reflectedand/or transmitted laser beam.

[0006] It is well known that an enhancement in surface field THzemission can be achieved from a variety of semiconducting materials whena large magnetic field is applied to it. The main effect of the Lorentzforce is to re-orientate the THz dipole with respect to the surface ofthe semiconductor, with little change to the dipole strength. This newdipole orientation allows a greater transmission of the THz radiationthrough the surface of the semiconductor which results in an increase inemitted THz power.

[0007] Provision of high power, large magnetic fields can be complex andexpensive. It would, therefore, be beneficial to achieve the inducedenhancement in surface field THz emission without the requirement of thelarge magnetic field.

[0008] As an alternative, or in addition to the use of a large magneticfield, THz emitters electrodes may be placed adjacent to the surface ofthe THz emitting semiconductor in order to create an electric fieldacross the surface. This technique allows a certain amount ofcontrollability to be achieved. However, it is limited by the break-downvoltage and in terms of the frequency that can be generated (<1.5 THz)as higher voltages will break down the field and the metal electrodeswill melt across the gap.

[0009] According to the present invention there is provided a device forgenerating terahertz radiation comprising:

[0010] a dipole generating layer;

[0011] a coupling block, transparent to laser light, in contact with thesurface of the dipole generating layer to couple light from a laser, inuse, to the surface of the dipole generating layer, in use; and

[0012] an extraction block, in contact with the surface of the dipolegenerating layer to provide an emission extraction surface, wherein therefractive indices of the dipole forming layer, the coupling block andthe extraction block are substantially equal, such that the dipolegenerated, in use, upon illumination of the dipole generating layer by alaser, has an axis which is not perpendicular to the emission extractionsurface.

[0013] The segment of the emission pattern that is extracted is notaligned with the dipole axis, therefore, as the deviation of the dipoleaxis from the perpendicular increases, the emission strength isenhanced.

[0014] The extraction block and the coupling block may be one of thegroup of a prism or a lens.

[0015] Suitable materials for the dipole forming layer include InAs andGaAs. Whereas suitable materials for the extraction block and couplingblock include InAs, GaAs, GaP and InP.

[0016] Where the incident laser is to be reflected at the surface of thedipole generating layer, the coupling block and the extraction block maybe provided as a single component. Where the incident laser is to betransmitted through the dipole generating layer, the dipole generatinglayer and the extraction block may be provided as a single component.

[0017] The dipole-forming layer may comprise at least onesemi-conducting layer. The dipole-forming layer may be a p-i-n diode.

[0018] The refractive index of the dipole forming layer may beeffectively reduced by forming holes in the emission surface, thisenables a greater segment of the dipole emission pattern to beextracted. The surface of dipole-forming layer may be patterned.

[0019] The surfaces of the coupling block and/or the extraction blockmay be coated with an anti-reflective material.

[0020] A system may be provided which comprises the aforementioneddevice for generating terahertz radiation, a laser generator which, inuse, generates sub-picosecond laser pulses and control circuitry.

[0021] Examples of the present invention will now be described withrespect to the accompanying drawings, in which:

[0022]FIG. 1 is an example THz emission pattern from a dipole within asemiconductor device;

[0023]FIG. 2A illustrates a physical construction according to thepresent invention where a prism provides the coupling and extractionblock and is used in combination with a terahertz emitting semiconductordevice;

[0024]FIG. 2B illustrates an alternative device according to the presentinvention utilising a wedge in combination with the terahertz emittingsemiconductor device; and

[0025]FIG. 2C illustrates an alternative device according to the presentinvention utilising a lens in combination with the terahertz emittingsemiconductor device;

[0026]FIGS. 3A, B and C illustrate the device of FIGS. 2A, B and C, inuse;

[0027]FIG. 4a illustrates a device according to the present invention,in use where the terahertz radiation is extracted, through the dipolegenerating layer, in the direction of the incident laser light;

[0028]FIG. 4b illustrates an alternative device of the present inventionwhere the extraction block and the dipole generating layer are providedby a single component; and

[0029]FIG. 5 illustrates the calculated emitted THz power as a functionof the dipole angle.

[0030] As discussed above, illumination of the surface of thesemiconductor device 3 by sub-picosecond laser pulses 4 with photonenergy greater than the semiconductor bandgap results in formation of adipole 1. This dipole 1 emits a coherent THz transient with a powerdistribution that is illustrated in FIG. 1. The axis 2 of the dipole 1is perpendicular to the surface of the semiconductor 3. The region ofthe emission pattern which may be extracted through this surface islimited by the refractive properties of the semiconductor material. In amaterial with a refractive index, n≈3.5 all radiation at an internalincidence angle of greater than approximately 17° to the normal of thesurface, will be reflected back within the material. The region definedby this angle is referred to as the emission cone. Since the emissionpattern of dipole 1 is biassed away from the axis of the dipole 2, itfollows that the restriction to, say, ±17° would prove to be limiting.It has been shown that only 0.1 of the emitted power will escape undersuch circumstances.

[0031] By introducing a block of additional material 5 of the samerefractive index the orientation of the dipole 1 will be unaffected. Thesurface 6 of this block 5 can be formed in a non-parallel plane to thatof the dipole forming layer 3 such that an alternative section of theTHz emission pattern is presented to the external surface 6. There willstill be a limitation of ±17° to the normal of this alternative surface6 but this will correspond to a larger angle relative to the dipole axis2, where the emission is stronger. Consequently, an enhancement of thelevel/efficiency of THz extraction can be found without the need for ahigh power, large magnetic field.

[0032] The spot size, to which the excitation laser 4 is focussed,determines the lateral extent of the THz dipole 1. If this spot size issmall (<100 microns), diffraction effects are important, and the THzradiation is emitted over a broad range of angles as shown in FIG. 5. Insuch a case a diffraction grating may be introduced onto the surface ofthe dipole forming layer 3 to allow for an increased level of extractionof the radiation through the surface.

[0033] It is preferable to produce a directed THz beam, as such it isnecessary to use a spot size larger than the THz wavelength (typicallyapproximately 250 microns). For a larger spot, the in-plane coherenceacross the emitting region tends to collimate the emitted THz radiationin the same direction as the reflected laser light. Thus, by increasingthe spot size, a greater friction of the THz power can be extracted. Theexcited spot can be considered as a distribution of point dipoles, withrelative phases set by the phase variation across the laser pulse,causing collimation in the reflection direction. Therefore, as the angleof incidence (and hence emission) is varied, the power obtained in thecollimated direction follows closely the emission pattern for the pointdipole, as has been previously demonstrated experimentally. This meansthat, even with a large laser spot, it remains highly beneficial tore-orient the dipole relative to the surface. By using a coupling block7 in the form of a prism 5 a, wedge 5 b or lens 5 c one must also allowthe collimated laser to excite the dipole forming layer 3 at an anglesuch that the direction of the emitted THz beam is in an efficientradiating direction for the dipole 1 and able to be coupled out of thedipole forming layer—extraction block combination. This can be achievedby passing the exciting laser 4 through another surface of theextraction block 5.

[0034] The terahertz generator of FIG. 2A is provided in combinationwith a 45° prism 5 a. The materials used for the different componentsare of substantially similar refractive indices. In this case theterahertz generator 3 is made from InAs (n=3.5) and the prism 5 a ismade from GaAs (n=3.5). The material of the prism 5 a must betransparent to the incident laser light 4.

[0035] The prism 5 a provides a means for coupling an incident laser 4to the surface of the terahertz generator 3 in addition to a secondaryextraction surface 6 for the dipole emission pattern as discussed above.In other words an extraction block 5 is provided such that theextraction surface 6 of the device is altered to be at 45° to theoriginal semiconductor 3 surface.

[0036] The device of FIG. 2B is similar to that of 2A but that theextraction block 5 is a wedge 5 b.

[0037] In FIG. 2C the extraction block 5 is provided by a lens 5 c,which allows access to an increased level of internal angles. FIGS. 3A,B and C illustrate these devices in use.

[0038]FIG. 4A shows a three component device of the present inventionfor use where the terahertz radiation 8 is to be transmitted through thedipole generation layer 3. A coupling block 7 is provided between theTHz generator 3 and the incident laser 4, in use. The dipole 1 is formedin the dipole forming layer 3 at the point where the laser 4 first comesinto contact with it (illustrated in FIG. 4A). The terahertz radiationis extracted from the dipole forming layer via an extraction block.Since this dipole generation location is not adjacent to the emissionsurface it is possible to reshape the dipole forming layer 3 (as shownin FIG. 4B) to allow it to act as an extraction block directly. In otherwords an alternative surface 6 can be formed within the THz generator 3such that the stronger section of the THz emission pattern can beaccessed and extracted.

[0039] The device when using a p-i-n diode is tunable, in use, byadjusting the voltage across the diode. Higher voltage leads to largercharge carrier acceleration and hence a larger maximum THz frequency (upto 40 THz). Surface field emitters would be tailored during manufactureto a particular frequency, by altering the doping concentrations duringsemiconductor growth, which controls the electric field profile.

[0040] An alternative way to match the refractive indicies is to reducethe index of the emission layer 3. By forming holes in the emissionsurface of the dipole layer, the effective refractive index is reduced,allowing matching to an extraction block 5 with lower refractive index.This requires the holes, and their separation, to be much smaller thanthe THz wavelength (perhaps 10 microns). Reducing the refractive indexincreases the range of internal angles which may be extracted, so thiswill also increase the output power in the absence of the extractionblock 5 (prism or lens).

[0041] In an alternative specific example of the present invention theprism is fabricated using molecular beam epitaxy to deposit a 500 nmthick layer of crystalline InAs onto a 500 μm semi-insulating (100) GaAswafer. The InAs layer is found to be n-type doped with a carrier densityof 5×10¹⁶ cm⁻³ at room temperature. The wafer is cleaved into 1.5 mmstrips which are glued, epilayer down, onto a brass polishing block. Twofacets of the prism are then polished with diamond pastes to form aright-angle prism. Finally, measurements are performed at roomtemperature, with the device suspended in air, and one of the unpolishedstrips is used as a (planar) control sample.

[0042] Pulses from a Ti:sapphire laser (wavelength=920 nm, TM polarised,pulse length=140 fs, pulse energy=2.5 nJ, pulse repetition rate 86 MHz)are incident at 45° on the back surface of the control and focussed toform a Gaussian intensity profile (σ=45 μm). The laser pulses have aphoton energy below the bandgap of GaAs, but above that of InAs, so thepulses pass through the GaAs before being absorbed in the InAs. The THztransient is thus generated in the InAs, but, because the index mismatchis small, there is negligible refraction when it passes into the GaAs.

[0043] The angular power distribution of THz radiation emitted from thecontrol sample is obtained by measuring the voltage of an aperturedGolay-cell bolometer (Cathodeon, Cambridge UK), which is rotated aroundthe sample. The resulting radiation pattern is shown in FIG. 6a. The twolobes on the polar plot show the weakly collimated THz beam propagatingin the direction of the reflected (45°) and transmitted (135°) laserbeam. Similar radiation patterns have previously been reported for bulksurface-field emitters. The measured power is identical to that obtainedfrom the surface of an InAs bulk wafer under the same excitationconditions.

[0044] The prism sample is excited under the same conditions as theplanar control, with the laser at normal incidence to one of thepolished facets, giving an internal angle at the InAs interface of 45°.The THz emission is then collimated around a direction 45° to the dipoleaxis, and emerges normal to the other prism facet. The measuredradiation pattern is shown in FIG. 6b. The peak THz angular intensity is8.7 times greater than for the planar reference sample. Furthermoreusing f/3 collection optics to collect THz radiation emitted within acone of 9.5°, the total power (≈7 μW) is found to be 21 times greaterfor the prism. Indeed, the angularly broad emission from the prismresults in considerably larger enhancement factors when using evensmaller f-number optics.

[0045]FIG. 7 shows the electric field of the THz transient produced bythe prism emitter. The shape of the THz pulse emitted from the prism issimilar to the weaker measurements obtained from the reference sampleand from a bulk InAs wafer under the same conditions. These resultsdemonstrate the coherent nature of the generated radiation, and showthat the prism does not modify the desirable half cycle waveform of theTHz pulse. The inset of FIG. 7 shows the power spectrum of the emittedpulse.

[0046] Other approaches have been used to produce high power pulses. Forexample, it has been known for several years that the power generated atsemiconductor surfaces can be enhanced by approximately an order ofmagnitude by the application of a large (3-8 Tesla) magnetic field. Thiseffect has been explained theoretically as a consequence of the rotationof the THz dipole, resulting from the Lorentz force acting on the movingcarriers. Our ability to produce a similar power enhancement using theprism geometry provides an experimental confirmation of thisexplanation. In contrast to the prism device, magnetic-field-enhancedemitters have the disadvantage that they require large magnets toproduce the high magnetic fields, which limits their use in manyapplications.

1. A device for generating terahertz radiation comprising: a dipolegenerating layer; a coupling block, transparent to laser light, incontact with the surface of the dipole generating layer to couple lightfrom a laser, in use, to the surface of the dipole generating layer, inuse; and an extraction block, in contact with the surface of the dipolegenerating layer to provide an emission extraction surface, wherein therefractive indices of the dipole forming layer, the coupling block andthe extraction block are substantially equal, such that the dipolegenerated, in use, upon illumination of the dipole generating layer by alaser, has an axis which is not perpendicular to the emission extractionsurface.
 2. A device according to claim 1, wherein the extraction blockis one of the group of a prism and a lens.
 3. A device according toclaim 1, wherein the coupling block is one of the group of a prism and alens.
 4. A device according to claim 1, wherein the dipole forming layeris made from a material from the group of InAs and GaAs.
 5. A deviceaccording to claim 1, wherein the extraction block and coupling blockare made from a material from the group of InAs, GaAs, GaP and InP.
 6. Adevice according to claim 1, wherein the incident laser is reflected atthe surface of the dipole generating layer.
 7. A device according toclaim 6, wherein the coupling block and the extraction block areprovided as a single component.
 8. A device according to claim 1,wherein the incident laser is transmitted through the dipole generatinglayer.
 9. A device according to claim 8, wherein the dipole generatinglayer and the extraction block are provided as a single component.
 10. Adevice according to claim 1, wherein the dipole-forming layer comprisesat least one semi-conducting layer.
 11. A device according to claim 10,wherein the dipole-forming layer is a p-i-n diode.
 12. A deviceaccording to claim 1, wherein the refractive index of the dipole forminglayer is effectively reduced by forming holes in the emission surface.13. A device according to claim 1, wherein the surface of dipole-forminglayer is patterned.
 14. A device according to claim 1, wherein thesurfaces of the coupling block are coated with an anti-reflectivematerial.
 15. A device according to claim 1, wherein the surfaces of theextraction block are coated with an anti-reflective material.
 16. Asystem comprising: a device for generating terahertz radiation accordingto claim 1; a laser generator which, in use, generates sub-picosecondlaser pulses; and control circuitry.